Waiting for a element to become visible

[Lord Nemesis]

We have a few HTML games in which we need to measure the loading time. The game loads many of the assets asynchronously and so the regular document load events captured by WPT might not be helpful.

Is it possible to measure the time till a certain button becomes visible in the page. Selenium typically has methods available to wait for visibility of an element, but I don't see similar options in WPT scripting language.